harden obiskio error handling with explicit variants and bounds checking

Extend `SKError` with `BadMagic`, `Truncated`, and `InvalidData` variants to replace `expect()` calls and unsafe slice indexing. Implement proper error chaining via `source()` and update `Display` formatting. Improve magic byte validation and serde error mapping for clearer debugging. Documents a broader roadmap for further crate hardening, including LRU eviction, concurrency, and benchmarking.

src/obiskio/src/unitig_index.rs

@@ -218,23 +218,31 @@ fn read_idx(path: &Path) -> SKResult<(Vec<u8>, Vec<u32>)> {
     let data = std::fs::read(path).map_err(SKError::Io)?;
     let mut pos = 0;
 
-    if &data[pos..pos + 4] != &MAGIC {
-        return Err(SKError::Io(std::io::Error::new(
-            std::io::ErrorKind::InvalidData,
-            "unitig index: bad magic",
-        )));
+    let magic_bytes = data.get(pos..pos + 4)
+        .ok_or(SKError::Truncated { context: "unitig index: magic" })?;
+    if magic_bytes != &MAGIC {
+        return Err(SKError::BadMagic {
+            expected: "UIDX",
+            got: magic_bytes.try_into().unwrap(),
+        });
     }
     pos += 4;
 
-    let n = u32::from_le_bytes(data[pos..pos + 4].try_into().unwrap()) as usize;
+    let n_bytes = data.get(pos..pos + 4)
+        .ok_or(SKError::Truncated { context: "unitig index: n_unitigs" })?;
+    let n = u32::from_le_bytes(n_bytes.try_into().unwrap()) as usize;
     pos += 4;
 
-    let seqls = data[pos..pos + n].to_vec();
+    let seqls = data.get(pos..pos + n)
+        .ok_or(SKError::Truncated { context: "unitig index: seqls" })?
+        .to_vec();
     pos += n;
 
     let mut packed_offsets = Vec::with_capacity(n + 1);
     for _ in 0..=n {
-        packed_offsets.push(u32::from_le_bytes(data[pos..pos + 4].try_into().unwrap()));
+        let off_bytes = data.get(pos..pos + 4)
+            .ok_or(SKError::Truncated { context: "unitig index: packed_offsets" })?;
+        packed_offsets.push(u32::from_le_bytes(off_bytes.try_into().unwrap()));
         pos += 4;
     }